qemu-iotests: Extend 041 for unbacked mirroring Add a new test case in file 041 for mirroring unbacked images in "absolute-paths" mode. This should work, if possible, but most importantly, qemu should never crash. Signed-off-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>